Presumably you're paying a monthly fee for your ISP, one which includes an IP of some sort. It doesn't cost anything for an ISP to give you a static address (which likely can be removed at their whim) rather than a dynamic address given you're likely to be needing an IP 24/7 (as you're the sort of person who knows what an IP address is)

If it costs $50 for a single IP, that would be 50 cents a month at a decent return on investment (and the value of that IP is still increasing), that is part of your monthly charge (which I suspect is far more than 50c).

Additionally, if you changed ISP you're going to lose that "investment" in your static IP, so you've basically paid $10 to lock yourself in.